1. Field of the Invention
The present invention relates to a signal emitter for optical fibers.
To be more precise, the present invention concerns an emitter of optical signals for optical fibers including an electroabsorption modulator downstream of a laser.
2. Description of the Prior Art
Two types of modulator are conventionally used to convert electrical signals into optical signals travelling in an optical fiber: Mach-Zhender modulators and electroabsorption modulators.
Mach-Zhender modulators are fragile and bulky modulators using crystals and require high electrical power at a voltage from 7 to 10 volts.
Electroabsorption modulators are semiconductor modulators, are less fragile and more compact, and require less electrical power at a voltage from 2 to 2.5 volts.
However, the phenomenon known as optical “chirp” is relatively high in this latter type of modulator. It distorts the rising and falling edges of the pulses of the optical signals because of spurious frequencies. Thus electroabsorption modulators are usable only for transmission over short distances, for example for interfaces between equipment units or local telephone networks.
In particular, transient “chirp” leads to distortion of normally squarewave optical signals by the formation of positive and negative peaks at the rising and falling transitions, which spread the frequency spectrum and generate intersymbol interference giving rise to propagation transmission problems.
Also other optical signal pulse distortion phenomena are due to defects of the electronics supplying the signal.
Consequently, there occur:                The “jitter” phenomenon corresponding to translation with respect to time of the rising and falling edges of the pulses of the optical signals;        Distortion due to the rising and falling times of said edges, causing them to become curved; and        A general asymmetry of the optical signals.        
These distortions occur in all modulators but the “chirp” phenomenon is particularly serious in electroabsorption modulators.
The present invention solves the above problem by proposing a hybrid modulator eliminating these spurious phenomena from the optical signals and enabling them to be used for transfers over long distances.